Optical modules using optical waveguides, particularly optical multiplexer/demultiplexer, are important optical parts used in many optical modules, including ones for wavelength division multiplexing communication. Recently, attempts have actively been made to constitute an optical multiplexer/demultiplexer by PLC for reduction in size and cost.
FIG. 10 is a plan view showing a basic construction of a conventional PLC type optical multiplexer/demultiplexer. The optical multiplexer/demultiplexer has a construction such that a wavelength selection filter 3 is inserted into a crossing portion of a Y-branch type optical waveguide comprising optical waveguides 1a, 1b and 1c. In FIG. 10, only a core portion of the optical waveguide is shown, which is embedded in a substrate portion including the optical waveguide. All of the drawings which follow the same figure also show only core portions. The wavelength selection filter 3 is fixedly inserted into a dicing groove 2 which is cut vertically into the surface of the substrate portion. Although in FIG. 10 a gap is shown between the groove 2 and an optical member of the filter 3, the gap is shown to make it easy to understand that the optical member 3 is inserted into the groove 2. Actually, the optical member is fixed by being inserted into the groove. The illustrated dimensions do not show concrete dimensions in a proportional relation. This is also the case with the succeeding drawings each showing like groove and optical member.
When light having a transmission wavelength of the wavelength selection filter 3 is incident on the above optical module using optical waveguides and light having a reflection wavelength is incident on the same module from the optical waveguide 1c, both light signals propagate through the optical waveguide 1a. In this way, this optical module acts as an optical multiplexer. If both light signals are incident from the optical waveguide 1a, their transmission wavelength light and reflection wavelength light propagate through the optical waveguide 1b and the optical waveguide 1c, respectively, so that this optical module acts as an optical demultiplexer. Thus, according to the illustrated construction it is possible to obtain a PLC type optical multiplexer/demultiplexer.
However, this construction involves the drawback in which an excess insertion loss occurs in the route of reflected light due to a positional deviation of the wavelength selection filter 3. This is because offset occurs between the light reflected by the filter and the optical waveguide on which the light is incident. Since the position of the wavelength selection filter is determined by the dicing groove, an excess insertion loss can be prevented by enhancing the positioning accuracy of dicing. In this case, however, a complicated process results, causing an increase in fabrication cost. Therefore, in the development of a PLC type optical multiplexer/demultiplexer, it is an important subject how an element of lessened insertion loss is to be fabricated in a simple manner.
The following proposals have been made heretofore for attaining the above-mentioned subject.
FIG. 11 shows a first example described in Japanese Patent Laid-open No. 6-174954 (Patent Literature 1). According to this example, in the vicinity of the wavelength selection filter 3, a spot size Ws of propagating light is made larger than in the other portion to suppress the influence of offset. The spot size Ws can be enlarged by either increasing or decreasing a core width W. One example of core width options described in Patent Literature 1 is shown in FIG. 11. FIG. 14 shows the relationship between the core width W and the spot size Ws of propagating light. In the same figure, the difference Δ in refractive index between core and clad is set at 0.84%, core thickness d is set at 4.5 μm, and operating wavelength is set at 1.3 μm. In this case, a cutoff core width (a maximum value of core width free of a higher mode) is 4.5 μm and Ws is 5 μm. Generally, in an optical circuit for communication, a core having a cutoff core width is used from the standpoint of maintaining a single mode characteristic and decreasing the radius of curvature. In this example, therefore, W of a portion sufficiently spaced away from the filter is set at 4.5 μm. It is seen from FIG. 14 that the value of Ws can be made equal to, e.g., 10 μm by setting W at 11 μm or 0.7 μm. Thus, it is seen that in this example the influence of offset is suppressed by adopting a tapered shape such that the core is widened from 4.5 to 11 μm or narrowed to 0.7 μm in the vicinity of the filter.
FIG. 12 shows a second example described in Japanese Patent Laid-open No. 8-190026 (Patent Literature 2). According to this example, in the vicinity of the wavelength selection filter 3, the width of the optical waveguide is made larger than in the other portion to suppress the influence of offset as in the above first conventional example. In this example, moreover, a gap is formed in the core crossing portion to prevent formation of an air bubble during fabrication of the optical waveguide. According to this example, it is also possible to prevent occurrence of an excess loss caused by an air bubble. FIG. 13 shows a third example described in Japanese Patent Laid-open No. 2000-180646 (Patent Literature 3). In this example, cores 1a, 1b and 1c are separated from one another to prevent the formation of an air bubble and the width of each core is narrowed in a tapered shape toward a wavelength selection filter to diminish an insertion loss.
The above conventional examples still involve the following drawbacks. First, in the above first conventional example, a dead-ended space is present between the cores 1a and 1b, so that such an air bubble 111 as shown in FIG. 15 is apt to occur at the time of forming a clad, and an excess loss caused by the air bubble is apt to occur. FIG. 16 shows the relationship between an excess loss caused by an air bubble and the core width W, in which the axis of ordinate represents the excess loss. The example of FIG. 16 is a calculation result obtained by a two-dimensional BPM (Beam Propagation Method). The model shown in FIG. 17 is used for calculating the excess loss. According to this model, first a zero-order eigen mode 114 is introduced and propagated into a rectilinear core 112 with which a square air bubble 111 is in contact. Next, an overlap between a light distribution 115 just after the zero-order eigen mode passing the air bubble 111 and the zero-order eigen mode 114 is calculated to determine an excess loss. Each side length of the square is set at 2 μm. The difference Δ in refractive index between the core 112 and clad 113, as well as the operating wavelength, are the same as in FIG. 14. From FIG. 16 it is seen that the excess loss caused by the air bubble poses no problem when the core width is large but is serious when the core width is small. For example, when W is set at 11 μm in an effort to widen Ws to 10 μm, the excess loss caused by the air bubble is less than 0.1 dB. However, if W is narrowed to 0.7 μm, there occurs an excess loss of 10 dB or more. This is because the leakage of light from the core becomes greater with narrowing of the core width and hence the air bubble becomes more easily influential. From this point it is seen that widening the core width is advisable in this example. However, in the case of widening the core width, a higher mode is apt to occur even according to the core shape or by a slight positional deviation of the filter. If a higher mode is present, a problem arise in that interference with the zero-order mode occurs, making the light output unstable.
In the above second example, the gap is formed to prevent formation of an air bubble. However, since the structure described therein involves widening the core width, the same problem as in the above first example arises also in this case.
In the above third example, since cores are separated from one another, between cores light is radiated into the clad and propagates. Further, a core-to-core distance dz is as large as about 60 μm. Thus, a problem arises in that both reflected light and transmitted light require a long radiation distance, with a consequent increase of loss. FIG. 18 shows the relationship between the core-to-core distance dz and radiation loss. It is here assumed that Ws and the optical axis on the output side are respectively coincident with those on the incident side and that the refractive index of the clad is 1.52 and the operating wavelength is 1.3 μm. In this case, if dz=60 μm, the radiation loss becomes 4 dB or more for Ws=5 μm (a spot size at cutoff core width). It is seen from the figure that this radiation loss can be decreased by enlarging Ws, but that at dz=60 μm the radiation loss is still 0.4 dB or more even if Ws is widened to 10 μm. Also in this case the loss can be decreased to a level less than 0.1 dB by enlarging Ws up to 20 μm. However, to enlarge Ws to 20 μm, it is necessary to control W to 0.5 μm or less as is seen from FIG. 14, thus requiring an advanced fabrication technique. When W is narrow, the taper length also becomes large in order to prevent the occurrence of radiation loss. Consequently, also drawback occurs in which the number of elements obtained per wafer is decreased.
Thus, even by the above conventional examples, it has so far been practically difficult to fabricate a PLC type optical multiplexer/demultiplexer with reduced loss in a simple manner.